1. Field of the Invention
The invention relates to a manually operated chuck for machine tools for machining of rotationally symmetrical workpieces involving cutting.
2. Description of the Prior Art
A chuck of the instant kind for lathes is disclosed in DE 2004 889, which relates to a base body in which three clamping jaws are disposed in a radially movable arrangement. The base body is releasably connected to a machine tool. The clamping jaws are moved to and fro in the base body by means of a driving ring, and are used for clamping rotationally symmetrical workpieces. The clamping jaws are arranged on the base body with a pitch angle of 120°.
Two of the three clamping jaws are directly driveably connected to the driving ring by means of a wedge bar, and are moved linearly in the base body between the particular wedge bar and the clamping jaw due to the helical gearing.
The driving ring is in a shape-locking or drivable connection with a threaded spindle inserted in the base body, wherein the threaded spindle is mounted in the base body at right angles to its longitudinal axis and can be moved in this arrangement. The threaded spindle can be operated manually from the outside, for example, by using a spanner as a tool, which accounts for the fact that a chuck of this kind is also referred to as a manually operated chuck, because the clamping force to be applied is generated manually using the tool.
One of the three clamping jaws is in a direct, drivable connection with the threaded spindle by means of a helically geared wedge bar, and is consequently moved by the movement of the threaded spindle, and synchronously with the two other clamping jaws.
It has proven to be, a disadvantage and a particular technical challenge in such manually operated chucks over the past decades that it is not possible optimally to centre the workpiece in relation to the longitudinal axis the base body. This is because there is a fault tolerance resulting from the manufacturing process between the individual components required for operating the clamping jaws, namely the threaded spindle, the driving ring, the corresponding wedge bars and their clamping jaws, as a result of which tolerance the clamping jaws cannot adequately achieve a central fixing of the workpiece. The presence of play leads to the permanent effect that the workpiece slips out of the set position during its rotation and machining, within a limit area of about 5 μm. It is only possible to achieve a lower fault tolerance than 5 μm at the cost of huge expenditure in production, as a result of which it is more cost-effective to manufacture chucks with a lower fault tolerance than chucks with a higher fault tolerance. However, the higher the fault tolerance, the greater the inaccuracies in metal-cutting machining on a workpiece.
The manually operated chucks of prior art do make it possible to compensate for the existing play between the individual components that drive the clamping jaws, e.g. the driving ring, the wedge bars and the threaded spindle, providing the workpieces involved are small. However, the larger the circumference of the driving ring, the larger the magnitude of the play between the driving ring and the individual components required for operating the clamping jaws. The bearing play can also be compensated in the case of lightweight workpieces, because workpieces of this kind do not exert any significant forces on the clamping jaws, with the effect that they may remain in their set position. Consequently, with heavy and large workpieces, it is not possible to compensate for the existing play between the components required for clamping the workpiece, with the effect that there is a permanent fault tolerance.
These bearing plays are inaccuracies which cannot be compensated for when clamping the workpiece, rather these inaccuracies are transferred to the clamping situation of the workpiece in such a way that a considerable inaccuracy arises between the position of the workpiece and the centre of the chuck, which leads to inaccuracies on the workpiece in the course of its machining. As a result, precise production and machining of the workpiece is not possible without further measures being taken.
It has proved to be a further disadvantage in the disclosed, manually operated chucks that clamping errors arise during the machining process which is often time consuming and highly complicated, because the machined workpiece becomes lighter during the machining process as a result of having material removed from it. Such working procedures involving material removal then result in the original position of the workpiece being changed in relation to the chuck and therefore in relation to the machine tool. A readjustment of the workpiece is often time-consuming and complicated to achieve.
The clamped workpieces are rotated by the machine tool in order to be machined. Therefore, in the case of a chuck with three clamping jaws offset at an angle of 120° in relation to one another, and in particular with exceedingly heavy and large workpieces of 20 tonnes inherent weight, for example, it has been observed that one clamping jaw which is located in a certain angular position of the chuck has to carry the weight of the workpiece exclusively, with the effect that it is not guaranteed that this clamping jaw can hold the workpiece reliably. Rather, the weight of the workpiece exceeds the clamping force of the individual clamping jaw, with the effect that it the jaw is forced outwards, the clamping force acting on the workpiece is eliminated. The workpiece is thus not reliably held in the chuck, and consequently its intended clamping position cannot be retained.